The present invention pertains to an optical device for scanning optical path length. In particular, the present invention relates to an optical device for scanning optical path length using a set of moving prisms.
An optical path length scanner is an important component used in many applications including, for example and without limitation, optical auto-correlation applications, optical coherent tomography (xe2x80x9cOCTxe2x80x9d), and optical coherent domain reflectometer (xe2x80x9cOCDRxe2x80x9d), to name a few. Scanning amplitude, linearity, duty cycle, and repetition rate are key parameters used to determine the efficacy of optical path length scanners. For example, in many applications it is desirable to have simultaneously a large scanning amplitude (for example,  greater than 1 mm), good linearity (for example,  greater than 95%), a high duty cycle (for example,  greater than 75%), and a high repetition rate (for example,  greater than 200 Hz). In addition, compactness and simplicity are among further concerns that relate to manufacturing of an optical path length scanner.
FIG. 3 shows a conventional optical path length scanner 310 as it is commonly used in a conventional interferometer 300. As is well known, optical path length scanner 310 comprises retro-reflector 311 that is back and forth along a linear path with a driving mechanism such as, for example and without limitation, a galvanometer or a driven voice coil (the driving mechanism is not shown in FIG. 3).
As further shown in FIG. 3, radiation beam 325 is directed to impinge upon retro-reflector 311, and is reflected thereby to produce returning beam 328. As is well known by those of ordinary skill in the art, returning beam 328 is substantially parallel to incident beam 325, regardless of its alignment with respect to retro-reflector 311.
Limitations with prior art optical path length scanner 310 stem from difficulty in driving retro-reflector 311 in a back and forth motion. For example, one limitation of back-and-forth driving mechanisms is that the repetition rate is typically below a hundred hertz (100 Hz) if a scanning amplitude in the millimeter range is required. Another limitation of back-and-forth driving mechanisms is that good linearity can be obtained only for a small portion of a cycle.
Various other designs for optical path length scanners have been reported in the prior art. One example of another design relates to a scanning optical delay device having a helicoid reflecting mirror that is disclosed in U.S. Pat. Nos. 5,784,186; 5,886,806, and 5,907,423 (inventors Wang et al.). A further example of another design relates to a grating-based, phase control, optical delay line that is disclosed in U.S. Pat. No. 6,111,645 (inventor Tearney et al.). A still further example of another design relates to a scanning optical delay line comprised of a rotating-parallelogram prism that is disclosed in an article entitled xe2x80x9cScanning delay line with a rotating-parallelogram prism for low-coherence interferometryxe2x80x9d by Giniunas et al. in Applied Optics, Vol. 38, No. 34, Dec. 1, 1999, pp. 7076-7079. A yet still further example of another design relates to a rapid depth scanner comprised of a rotating cube that is disclosed in an article entitled xe2x80x9cRapid and scalable scans at 21 m/s in optical low-coherence reflectometryxe2x80x9d by Ballif et al. in Optics Letters, Vol. 22, No. 11, Jun. 1, 1997, pp. 757-759. However, none of these designs provide a practical optical path length scanner (i.e., an optical path length scanner having relatively low cost, having a long lifetime, and requiring little alignment) which has a repetition rate in the kilohertz range, and a scanning amplitude in the millimeter range with good linearity and a high duty cycle.
In light of the above, there is a need for an optical path length scanner capable of having a relatively high repetition rate (for example, up to the kilohertz range) and a scanning amplitude up to the millimeter range with good linearity and a relatively high duty cycle.
Embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide an optical path length scanner. Specifically, in accordance with a first embodiment of the present invention, an optical path length scanner comprises: (a) a set of prisms mounted evenly along a movable carrier; and (b) a mechanism that drives the movable carrier to move. In addition, in accordance with a second embodiment of the present invention, the optical path length scanner further comprises a first prism held stationary relative to a predetermined direction and arranged in a complementary orientation and position with respect to the set of prisms. In further addition, in accordance with a third embodiment of the present invention, the optical path length scanner further comprises a mechanism that applies a beam of radiation at a minimum deviation angle of incidence to the prisms.